As in many such cases, my dictionaries eschew responsibility by mentioning both spellings linked by "or". It seems, though, that in all such cases the ending -y is a well-established standard.

The popular "Bonnie Prince Charlie" in this spelling may be the source of two possible exceptions, whence the association with Scotland. It may well have been a whim by biographers, perhaps even driven by the purpose to create a Scottish idiosyncrasy (- my wild guess from googling, not expertise.)

However, from time to time we encounter back-formations from plural spellings, so-called Quaylisms. Example: "Kudos to the Brits, but not a single kudo to Cameron!"